1. Field of the Invention
This invention relates to the field of strain gage load cells.
2. Description of the Prior Art
A typical prior art high capacity bonded strain gage load cell utilizes a simple column as the load bearing member. Compressive loads on such a column result in axial compressive strains, and accompanying circumferential tensile strains. The circumferential tensile strain is of smaller magnitude than the axial strain, being related to the axial strain by Poisson's ratio as is well known in the art. Poisson's ratio for steel is about 0.3. Typically, one pair of strain gages are bonded to the column with their axes parallel to the column axis and another pair bonded with their sensitive axes transverse. The four gages are connected into a bridge arrangement, which then has an output substantially proportional to applied load.
While the above described load cell provides a robust and reliable means of measuring loads, there are two basic shortcomings, which have hampered workers in the field for many years.
As noted above, in the typical prior art load cell, the transverse gages experience only about 0.3 of the strain of the longitudinal gages, resulting in an output of only about 65% of the potential output of the sensing bride. Since the maximum output of high accuracy load cells is only about 2-3 millivolts/volt of excitation, any loss in output is undesirable, and much effort has been expended in past years to device structures to overcome this limitation. The structures so devised have, in general, been complicated and expensive to produce.
Another difficulty with prior art devices is even more troublesome for high accuracy cells. For reasons that appear to be connected with the increase in diameter of the column as the cell is loaded, and perhaps also because of changes in modulus of elasticity due to stress, the standard column type load cell displays a nonlinear output with respect to load of the order of 0.2%. Since high accuracy cells must have accuracies better than 0.2%, various electrical means have been utilized to overcome this deficiency. For example, U.S. Pat. No. 3,228,240 issued to A. N. Ormond, describes additional strain gages connected to the bridge circuit to compensate for such nonlinearity. This solution is not entirely satisfactory since it requires extra electrical components and additional installation labor result in extra cost and a reduction in reliability.